This invention relates to an electronic component-use package of surface mounting type, and a piezoelectric device comprising the same.
Conventionally, electronic component-use packages of surface mounting type are typically used in electronic devices, specific examples of which may be piezoelectric devices of surface mounting type, including crystal vibrators, crystal filters, and crystal oscillators.
The piezoelectric device is structurally characterized in that a piezoelectric vibration piece having metal thin-film electrodes formed on a surface thereof is hermetically sealed in an electronic component-use package to protect these electrodes from exposure to outside air.
For easy processing and reasonable cost, circuit boards to be mounted with piezoelectric devices may typically be generally called glass epoxy substrates including mesh glass fibers impregnated with epoxy resins. The circuit board further includes solder paste applied by, for example, screen printing to an upper part of its electrode pattern. The package of the piezoelectric device has terminal electrodes, and these terminal electrodes are stacked in layers on the electrode pattern of the circuit board. Then, the circuit board is placed in a fusion furnace (or heating furnace) to melt the solder paste and thereby solder-bond the piezoelectric device to the circuit board.
However, the package and the circuit board may have different coefficients of thermal expansion. This may generate stress that acts upon the solder joining the package to the circuit board, possibly causing the solder to crack. This problem may be obvious with any piezoelectric devices using ceramic-made, for example, alumina-made packages in combination with glass epoxy substrates, and even more obvious with heat-resistant applications, typically, in-vehicle piezoelectric devices having their packages and circuit boards be exposed to high temperatures. Such a severe environment even further elevates the coefficient of thermal expansion of the circuit board as compared to the coefficient of thermal expansion of the package, easily incurring fatigue failure of the solder.
The occurrence of cracks, which is barely a serious issue to be taken into account at normal temperatures, may be more noticeable at high temperatures. In addition to this problem, an impact, if imposed on the package and the circuit board, may trigger abrasion in any cracked part of the solder.
The applicant of this application found and disclosed some technical solutions to these problems (for example, JP 2006-32645 A).